“Surface” positioning systems to determine absolute coordinates (longitude, latitude, altitude) at any point on the earth's surface or in its atmosphere are already available on the market. The latest generation systems offer positioning by a network of satellites in geostationary orbits emitting hyper frequency signals. A moving body on the surface or in the earth's atmosphere and equipped with appropriate device can decode said signals and extract its own absolute coordinates, for example in longitude, latitude and altitude, from them. For example, these positioning systems include the American Global Positioning System (GPS) and the future European Galileo system.
All systems marketed at the moment provide the possibility of determining an absolute geographic position with sufficient precision for most purposes, for the purchase of an inexpensive and compact device.
On the other hand, since the operating principle of these systems is based on the use of microwaves, they cannot be used for immersed moving bodies at a depth of more than one meter.
Independently of “surface” positioning systems described above, the market offers positioning systems enabling an immersed moving body to determine its position relative to a given point, usually marked by a beacon. These systems use acoustic wave propagation and operate on the basis of the known “homing” principle. According to this principle illustrated in FIG. 1, each moving body (divers in this illustration) is equipped with a device that can be used at any time to determine the distance D separating it from the beacon and the bearing θ in which said beacon is located with respect to its velocity vector. For example, the beacon may be immersed from a boat that then forms a fixed point coincident with the location from which the moving body started.
The most efficient devices that can be used by immersed moving bodies are provided with an omni-directional acoustic antenna to determine the bearing θ of an incident acoustic wave at any time. Such a device developed for pleasure diving is described particularly in international patent PCT/IB01/01361 deposited by the XIOS S.A. company and published under reference WO 02/10793.
With a positioning system operating according to the “homing” principle, a moving body only knows its position relative to the beacon, and when said moving body movesalong both of the 3 axes of space, the distance D and bearing θ information provided by the device is inherently ambiguous: all they permit is to position the moving body on a sphere of diameter D centred on the beacon, as shown on FIG. 2.
Therefore, these “homing” positioning systems do not satisfy the need that an immersed moving body may have to determine its absolute geographic position, for example during submarine exploration missions, or archaeological excavations or prospecting campaigns.
A device compatible firstly with a GPS type “surface” positioning system and secondly a “homing” positioning system as described above that could be envisaged to satisfy this need, could operate as follows:                Before the immersion of the moving body, the device could determine absolute geographic position of the beacon that materialises the start point of the immersion of said moving body, for example using GPS, and memorising said position.        During the immersion, the device could determine the absolute geographic position of the moving body by combining the memorised position of the start point with the relative position of said moving body with respect to the beacon, supplied permanently by the positioning system by “homing”.        
However, this determination will be affected by the spherical ambiguity caused by positioning by “homing”.
One solution to satisfy the same need for an immersed moving body to determine its absolute geographic position, would be for said moving body to estimate its depth, to rise to the surface along a perfectly vertical rising path and then determining its absolute geographic position at the point on the surface, for example using GPS information. Apart from its tedious nature, this type of solution, which in any case is not very useful if the moving body is a diver, is only possible if said moving body can control its drift as it rises to the surface. Otherwise, the measurement made will still comprise an uncertainty.